Fuel processing system having gas recirculation for transient operations

ABSTRACT

A fuel processor system capable of circulating fuel processor system gases, e.g. reformate, anode exhaust, and/or combustor exhaust, through a fuel processor provides advantages. The system fuel cells discharge hydrogen-containing anode exhaust and oxygen-containing cathode exhaust, The fuel processor converts hydrogen-containing fuel to hydrogen-containing reformate to fuel the fuel cells. A catalytic combustor coupled with a vaporizer reactor is positioned in series downstream from the fuel cells. A bypass passage connects an outlet of at least one of the fuel processor, the fuel cells, the catalytic combustor, and the vaporizer reactor to the inlet of the fuel processor. The bypass passage is operable to circulate a fuel processor system gas to the inlet of the fuel processor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/963,248 filed Oct. 12, 2004 (now U.S. Pat. No. 7,442,461, issued Oct.28, 2008), which is a divisional of U.S. patent application Ser. No.10/055,101 filed Jan. 22, 2002 (now U.S. Pat. No. 6,838,200, issued Jan.4, 2005). The entire disclosures of each of these applications areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to fuel processors and, moreparticularly, relates to a fuel processor system having gasrecirculation for improved startup, shut down, turn down, and transientoperation.

BACKGROUND OF THE INVENTION

As is well known to those skilled in the art, in order to heat rapidlythe mass of a fuel processor to its proper operating temperature duringa startup cycle, it is preferable to provide the largest possibleheating gas flow therethrough. However, using fuel rich-combustion gasflow may exceed the temperature limits in the earlier stages of the fuelprocessor, thereby requiring additional stages to fully heat theremaining stages of the fuel processor.

During a shut down cycle, it is desirable to remove water from the fuelprocessor so that the water does not condense onto the catalysts whenthe fuel processor completely cools, which may damage the catalysts.Furthermore, it is also desirable to stop the fuel processor in apressurized state so that when the fuel processor cools and the gasescontract, the pressure the fuel processor remains above atmosphericpressure so that air is not drawn into the fuel processor. Conventionalshut down methods cannot continue operating without water injection, asthe ATR catalyst would get too hot.

During a turn down cycle, it is preferable to circulate a larger flow sothat the residence times within the reactors are more constant. However,in conventional fuel processors, as the power level is turned down theflow is thus reduced and the residence times in each reactor increases.This increase in residence times may lead to auto-ignition in the inlet,reverse water gas shift in the PrOx, cell reversal in fuel cell stackdue to non-uniform flow distribution of hydrogen containing reformate,and water collection in fuel cell stack.

During a transient cycle, it is preferable to have a constant flowthrough the reactors such that the pressure in the reactors remainsgenerally constant, thereby minimizing the lag in transient responseassociated with filling or venting volumes of the fuel processor.

Accordingly, there exists a need in the relevant art to provide a fuelprocessor that is capable of rapid thermal start without the complexityof multiple stages or risk of oxygen exposure. Furthermore, there existsa need in the relevant art to provide a fuel processor that, during shutdown, is capable of minimizing water in the reformate and be shut downat an elevated pressure to minimize condensation on the catalyst and airingestion upon cooling. Still further, there exists a need in therelevant art to provide a fuel processor that, during turn down, iscapable of minimizing auto-ignition in the inlet, reverse water gasshift in the PrOx, cell reversal in fuel cell stack due to non-uniformflow distribution of hydrogen containing reformate, and water collectionin fuel cell stack. Yet further, there exists a need in the relevant artto provide a fuel processor that, during transient operation, is capableof maintaining a generally constant flow rate through to the fuelprocessor to minimize the lag time associated with filling or ventingvolumes of the fuel processor. Still further, there exists a need in therelevant art to provide a fuel processor that is capable of operatingwithout water injection.

SUMMARY OF THE INVENTION

A fuel processor system capable of recirculating fuel processor systemgases, such as reformate, anode exhaust, and/or combustor exhaust,through the fuel processor to provide a number of distinct advantages isprovided. A fuel processor is also provided for converting ahydrogen-containing fuel to H₂-containing reformate. The fuel processorsystem may also include a plurality of fuel cells discharging anH₂-containing anode effluent and an O₂-containing cathode effluent. Acatalytic combustor is positioned in series downstream from theplurality of fuel cells and a vaporizer reactor is coupled to thecatalytic combustor. A bypass passage interconnects an outlet of atleast one of the group consisting of the fuel processor, the fuel cell,the catalytic combustor, and the vaporizer reactor to the inlet of thefuel processor. The bypass passage is operable to recirculate a fuelprocessor system gas to the inlet of the fuel processor.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a schematic view illustrating a fuel processor systemaccording to a first embodiment of the present invention;

FIG. 2 is a schematic view illustrating a fuel processor systemaccording to a second embodiment of the present invention;

FIG. 3 is a schematic view illustrating a fuel processor systemaccording to a third embodiment of the present invention; and

FIG. 4 is a schematic view illustrating a fuel processor systemaccording to a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses. For example, the present invention ishereafter described in the context of a fuel cell fueled by reformedgasoline. However, it is to be understood that the principles embodiedherein are equally applicable to fuel cells fueled by other reformablefuels. Furthermore, the present invention hereafter described in thecontext of a self contained fuel cell system having a reforming systemand a fuel cell system. However, it is to be understood that theprinciples embodied herein are equally applicable to a reforming systemonly.

Referring to FIG. 1, a fuel processor system, generally indicated as 10,according to a first embodiment of the present invention is illustrated,which provides rapid startup capabilities. Fuel processor system 10generally includes a fuel processor 12, a fuel cell stack 14, acatalytic combustor reactor 16, and a vaporizer reactor 18. Fuelprocessor 12 would typically include a primary reactor 12.2 such as asteam reformer or an autothermal reformer, a water gas shift (WGS)reactor 12.4 and a preferential oxidation (PrOx) reactor 12.6.

Fuel processor system 10 is arranged such that a first fuel inlet stream20 and a first water inlet stream 22 are introduced into fuel processor12 to produce a reformate stream 24 according to conventionalprinciples. During a startup cycle, an anode bypass valve 26 directsreformate stream 24 to an anode bypass passage 28. It is necessary toinitially bypass fuel cell stack 14 until “stack grade” (having COcontent less than about 100 ppm) reformate is produced. In order toproduce such stack grade reformate, it is necessary to heat the variouscomponents of fuel processor system 10 to their respective operatingtemperatures. Recirculated reformate in passage 30 from anode bypasspassage 28 is drawn into a recirculation compressor 32 together with afirst inlet air stream 34.

First fuel inlet stream 20 is then introduced into fuel processor 12.Reactions may be initiated in fuel processor 12 via a spark lit burneror by an electrically heated catalyst section (not shown). Heat producedby the reaction of first fuel inlet stream 20 and first inlet air stream34 warms fuel processor 12. First fuel inlet stream 20 and first inletair stream 34 are introduced in proportions slightly rich ofstoichiometric. This ensures that there is no excess oxygen, which coulddamage the catalysts within fuel processor 12. Ordinarily, reactionsnear stoichiometric conditions produce damagingly high temperatures;however, with a large excess of recirculated reformate 30 acting as adiluent, the gas temperature within fuel processor 12 is maintained atan appropriate level.

A portion, generally indicated at 36, of the flow through anode bypasspassage 28 is exhausted to catalytic combustor reactor 16. Under steadyflow, this exhausted reformate 36 is equal to the total mass flow offirst fuel inlet stream 20, first inlet air stream 34, first water inletstream 22 and vaporizer steam 38 that passes through fuel processor 12.This exhausted reformate 36 is reacted with a second inlet air stream 40in catalytic combustor reactor 16. Second inlet air stream 40 isdirected to catalytic combustor reactor 16 via a stack air compressor42, a cathode bypass valve 44, a cathode bypass passage 46, and anexhaust passage 48. Second inlet air stream 40 is bypassed around fuelcell stack 14 during startup to prevent drying of the membranes withinfuel cell stack 14. Heat from the reaction in catalytic combustorreactor 16 is integrated back into fuel processor 12 by vaporizingsecond water inlet stream 50 in vaporizer reactor 18 to producevaporizer steam 38, which typically is delivered to the PrOx-vaporizeror steam lines within fuel processor 12. Exhaust gases from combustor 16exits vaporizer reactor 18 through exhaust outlet 66.

During the startup cycle, the fuel and air are completely consumed(stoichiometric conditions) for maximum heat release within fuelprocessor system 12 for rapid heating without excessively hightemperatures. However, it is important to note that the temperaturewithin the PrOx 12.6 may initially be relatively high at about 357° C.However, once the PrOx is heated, normal operation is such that coolingof the PrOx according to conventional methods can be used.

Referring again to FIG. 1, once the various reactors within fuelprocessor 12 are warmed to their operating temperature, anode bypassvalve 26 routes reformate stream 24 to fuel cell stack 14 via passage52. Second inlet air stream 40 is then directed by cathode bypass valve44 to the cathode side of fuel cell stack 14 via passage 54. Thehydrogen from reformate stream 24 reacts with the oxygen from second airinlet stream 40 across a membrane electrode assembly within fuel cellstack 14 to produce electricity. Anode exhaust or stack effluent 56 fromthe anode side of fuel cell stack 14 includes a portion of hydrogen thatis directed back to catalytic combustor reactor 16 to provide heatrecovered in vaporizer 18. Cathode exhaust 58 from the cathode side offuel cell stack 14 includes oxygen also for use in catalytic combustorreactor 16. Anode exhaust 56 and cathode exhaust 58 are combined inexhaust passage 48 and react in catalytic combustor reactor 16.Vaporizer reactor 18 continues to provide vaporizer steam 38 to fuelprocessor 12. Note that the PrOx air, within fuel processor 12, is drawnfrom recirculation compressor 32 which contains only first inlet airstream 34 when anode bypass valve 26 directs reformate stream 24 to fuelcell stack 14. Preferably, a reformate check valve 60 is disposed inexhausted reformate passage 36 to ensure that anode exhaust 56 andcathode exhaust 58 in exhaust passage 48 are not drawn into fuelprocessor 12 by recirculation compressor 32.

As is well known in the art, catalysts, such as that which is often usedin water gas shift reactors (i.e. CuZn), are often sensitive to oxygenand condensed water. Therefore, this is particularly important aftershut down when the fuel processor cools and any water vapor condenses.That is, the reformate gases within fuel processors often have a veryhigh water (steam) content (typically 30%), which condense when the fuelprocessor cools after shut down. Additionally, as the fuel processorcools the condensation of water and the cooling of gases within the fuelprocessor may cause a reduction in gas pressure sufficient to pull avacuum even if valves at the inlet and exit seal a fuel processor. Atthis point, any leaks present in the various valves, fittings, orflanges may allow air into the fuel processor and potentially damage thewater gas shift catalyst. Therefore, additional features are illustratedin FIG. 2 to address these shut down issues.

The fuel processor system 10′, shown in FIG. 2, is the same as thatdescribed in reference to FIG. 1, where like reference numerals are usedto indicate like components. Referring to FIG. 2, a recirculation valve102 is positioned in recirculated reformate passage 30 and an exhaustvalve 104 is positioned in exhaust reformate passage 36. Recirculationvalve 102 and exhaust valve 104 are used in conjunction to control therecirculation ratio (i.e., the ratio of recirculated reformate stream tothe total reformate stream). That is, by opening recirculation valve 102the flow of recirculated reformate 30 is increased, while openingexhaust valve 104 the flow of recirculated reformate 30 is decreased.Furthermore, opening both valves 102, 104 decreases the pressure withinfuel processor 12. Recirculation valve 102 and/or exhaust valve 104 maybe closed to prevent anode exhaust 56 and cathode exhaust 58 from beingdrawn into fuel processor 12 by recirculation compressor 32.

The transition to normal operation for fuel processor system 10′, shownin FIG. 2, is the same as described in reference to FIG. 1.

Fuel processor system 10′, shown in FIG. 2, further provide a means toshut down fuel processor 12 without water condensation or air ingestion.For shut down, reformate stream 24 is circulated to anode bypass passage28 via anode bypass valve 26. Exhaust valve 104 remains closed to causehigher pressures within fuel processor 12. Recirculation valve 102 isthen slightly opened to maximize pressure within the capacity ofrecirculation compressor 32. During shut down, water is condensed andseparated from reformate stream 24 in a condenser 106, which isconnected to the system coolant loop (not shown). In normal operation,condenser 106 is used as an anode pre-cooler before fuel cell stack 14.

To further increase the pressure within fuel processor 12 during shutdown, recirculation compressor 32 draws in first inlet air stream 34.Preferably, the inlet to recirculation compressor 32 and the downstreamside of circulation valve 102 are small in volume such that afterrecirculation compressor 32 is stopped, the pressure will remain high.Subsequently, the oxygen within first inlet air stream 34 will reactwith the hydrogen in recirculated reformate 30 within fuel processor 12to produce additional heat, thereby increasing the pressure within fuelprocessor 12. However, if necessary, additional fuel from first fuelinlet stream 20 may be added during shut down to consume the oxygen infirst inlet air stream 34 in order to provide sufficient reactants (H₂and CO) within fuel processor 12. An oxygen sensor 108 is used in thefuel processor 12 as feedback to ensure that excess oxygen is notpresent. If the pressure within fuel processor 12 is higher than apredetermined level, exhaust valve 104 may be opened to reduce suchpressure.

Once the water has been condensed from reformate stream 24 and a highpressure condition has been achieved within fuel processor 12, fuelprocessor air mass flow controller 62 is closed to seal the inlet, anodebypass valve 26 remains in the bypass position, and exhaust valve 104remains closed to seal the exit. Recirculation compressor 32 is thenstopped. The resident gases within fuel processor 12 are dry and at anelevated pressure, which is desired for shut down condition,particularly with a CuZn water gas shift catalyst.

During the shut down cycle, the fuel and air are completely consumed(stoichiometric conditions) without water injection and withoutexcessively high reactor temperatures to allow the gases to be dried bycondenser 106. Back pressure regulator 214 is described below inconjunction with fuel processor system 10.

As is well known in the art, conventional fuel processors suffer fromvarious disadvantages when operating at reduce power and reduced flow,such as auto-ignition in the inlet, reverse water gas shift in the PrOx,cell reversal in the fuel cell stack, and water collection in the fuelcell stack. Furthermore, the transition between power levels are oftenslow to react due to the time necessary to pressurize or vent reactorvolumes so as to achieve steady flow conditions at the new power level.

Within the primary reactor temperatures in the inlet region increasesuch that there is a limited amount of time before undesirableauto-ignition of the fuel will occur. As the flow through the fuelprocessor is reduced at low power, the residence time within the inletis increased. Thus, the rate of reduction in flow and power is limitedby the auto-ignition condition in the inlet.

Within the PrOx reactor, after the oxygen is consumed, reformate that isexposed to catalyst will undergo reverse water gas shift reactions,thereby consuming desirable H₂ and creating undesirable CO. At reducedflow, the oxygen is consumed earlier in the PrOx reactor, therebyleaving a larger section of catalyst and a longer residence time forreverse water gas shift reactions to occur.

Within the fuel cell stack, the current flow through each fuel cell islimited by the fuel cell provided the lowest quantity of H₂. That is,the fuel cell with the lowest H₂ flow limits the current through all ofthe remaining fuel cells. Therefore, a portion of the available quantityof H₂ (typically 10 to 20%) leaves the fuel cell stack unused. Atreduced flows, the portion of H₂ leaving the fuel cell stack needs to behigher for stable operation, which is likely the result of less uniformflow distribution at reduced flows. Also contributing to the minimumflow for stable fuel cell stack operation is the need to clear condensedwater to prevent it from collecting in and blocking passages within thegas distribution plates.

In conventional systems, the flow rate through the fuel processor systemvaries with power level, thus the associated pressure drop necessitatesa change in reactor pressure between power levels. However, a change inreactor pressure requires time for flow to fill or vent to thedownstream reactors in order to achieve the steady pressure at the newpower level. The numerous aforementioned disadvantages are overcome inthe present invention by maintaining a higher flow rate, even during lowpower operation, by recirculating gases through the fuel processor andstack.

Fuel processor system 10″, shown in FIG. 3, illustrates a system havingreformate circulation through the fuel processor for startup, means forwater condensation and pressurization for shut down, and circulationthrough the fuel processor and anode for turn down and transients. Thefuel processor system 10″, shown in FIG. 3, is the same as thatdescribed in reference to FIGS. 1 and 2, where like reference numeralsare used to indicate like components.

More particularly, for startup, anode bypass valve 26 directs reformatestream 24 to anode bypass passage 28. First fuel inlet stream 20 isintroduced into fuel processor 12. First inlet air stream 34 isdelivered to fuel processor 12 by a fuel processor air compressor 202.FIG. 3 shows first inlet air stream 34 being delivered to threelocations in fuel processor 12 in the form of POx air stream 204, startair stream 206 and PrOx air stream 208. POx and PrOx air streams 204,208 would normally be part of fuel processor 12. Heat produced by thereactions of fuel inlet stream 20 and inlet air stream 34 warms fuelprocessor 12. By staging the inlet air to provide multiple heatinglocations, the startup time is reduced by improving heat distributionwithin fuel processor 12.

To initiate reactions in each of these locations, a spark lit burner oran electrically heated catalyst section (not shown) is used. The overalloxygen to carbon (o/c) ratio (i.e. ratio of first inlet air stream 34 tofirst fuel inlet stream 20) is introduced in proportions slightly richof stoichiometric to ensure that no excess oxygen is present, whichcould damage the catalyst within fuel processor 12. The recirculatedreformate 30 acts as a diluent so that all the available first inlet airstream 34 is reacted without excessively high temperatures within fuelprocessor 12.

Exhaust reformate passage 36 is employed to exhaust excess reformate tocatalytic combustor reactor 16. Under steady flow, this exhaustedreformate in passage 36 is equal to the total mass flow of first fuelinlet stream 20, first inlet air stream 34, first water inlet stream 22and vaporizer steam 38 that passes through fuel processor 12. Thisexhausted reformate in passage 36 is reacted with second inlet airstream 40 in catalytic combustor reactor 16. Second inlet air stream 40is directed to catalytic combustor reactor 16 via stack air compressor42, cathode bypass valve 44, cathode bypass passage 46, and exhaustpassage 48. Second inlet air stream 40 is bypassed around fuel cellstack 14 during startup to prevent drying of the membranes within fuelcell stack 14. Heat from the reaction in catalytic combustor reactor 16is integrated back into fuel processor 12 by vaporizing second waterinlet stream 50 in vaporizer reactor 18 to produce vaporizer steam 38,which typically is delivered to the PrOx-vaporizer or steam lines withinfuel processor 12. An anode check valve 210 and a cathode check valve212 are shown to prevent back flow of reformate exhaust 48 into fuelcell stack 14. Preferably, a reformate check valve 60 is also disposedin exhausted reformate passage 36 to ensure that anode exhaust 56 andcathode exhaust 58 in exhaust passage 48 are not drawn into fuelprocessor 12 by recirculation compressor 32.

Once the various reactors within fuel processor 12 are warmed to theiroperating temperature, anode bypass valve 26 routes reformate stream 24to fuel cell stack 14 via anode inlet passage 52. Second inlet airstream 40 is then directed by cathode bypass valve 44 to the cathodeside of fuel cell stack 14 via cathode inlet passage 54. The hydrogenfrom reformate stream 24 reacts with the oxygen from second air inletstream 40 across a membrane electrode assembly within fuel cell stack 14to produce electricity. Anode exhaust or stack effluent 56 from theanode side of fuel cell stack 14 includes a portion of hydrogen that isdirected back to catalytic combustor reactor 16 where it is oxidized toprovide heat. Cathode exhaust 58 from the cathode side of fuel cellstack 14 includes oxygen which may also be used in catalytic combustorreactor 16. Anode exhaust 56 and cathode exhaust 58 are combined inexhaust passage 48 and react in catalytic combustor reactor 16.Vaporizer reactor 18 continues to provide vaporizer steam 38 to fuelprocessor 12.

A back pressure regulator 214 is used to set the pressure within fuelprocessor system 10″, while recirculation compressor 32 determines theamount of reformate recirculated. As additional flow from first fuelinlet stream 20, first inlet air stream 34, first water inlet stream 22,and vaporizer steam 38 is added to fuel processor 12, additionalreformate flow will split to exhausted reformate passage 36 to maintainthe system pressure. Therefore, at high power, the system 10″ operatesat a low recirculation ratio, whereby a larger portion of reformatestream 24 is “fresh” having a relatively high H₂ content. At low power,the system 10″ operates at a high recirculation ratio, whereby a largerportion of reformate stream 24 is re-circulated and having a relativelylow H₂ content. It is important to note that recirculation compressor 32according to the present embodiment need only overcome the pressure dropthrough fuel processor 12 and fuel cell stack 14 during normaloperation, unlike the system shown in FIG. 2 where the pressure woulddrop to atmospheric pressure downstream of recirculation valve 102 toallow first inlet air stream 34 to be drawn in. To this end, fuelprocessor system 10″ illustrated in FIG. 3 requires an additional fuelprocessor air compressor 202. Alternatively, stack air compressor 42 canbe used to deliver air to fuel processor 12.

As best seen in FIG. 3, fuel processor system 10″ maintains a flow ratethat is approximately equal to a fuel processor system operating at anoptimum power level. This higher flow rate helps overcome many of thedisadvantages described above.

During the shut down cycle of fuel processor system 10″, anode bypassvalve 26 routes reformate stream 24 to anode bypass passage 28. Secondinlet air stream 40 is then directed by cathode bypass valve 44 throughcathode bypass passage 46 to catalytic combustor reactor 16. This willprovide air to catalytic combustor reactor 16 to react with anyexhausted reformate in passage 36 from the recirculation loop.

Backpressure regulator 214 is adjusted to indirectly produce the highestpossible pressure within the capacity of recirculation compressor 32. Asreformate stream 24 recirculates through fuel processor 12, water iscondensed and separated in condenser 106.

To further increase the pressure within fuel processor 12 prior to shutdown, fuel processor air compressor 202 draws in first inlet air stream34. Subsequently, the oxygen within first inlet air stream 34 will reactwith the hydrogen in circulated reformate 30 within fuel processor 12 toproduce additional heat, thereby increasing the pressure within fuelprocessor 12. However, if necessary, additional fuel from first fuelinlet stream 20 may be added during shut down to consume the oxygen infirst inlet air stream 34 in order to provide sufficient reactants (H₂and CO) within fuel processor 12. An O₂ sensor 108 is used in fuelprocessor 12 as feedback to ensure that excess oxygen is not present.

Once the water has been condensed from reformate stream 24 and a highpressure condition has been achieved within fuel processor 12, fuelprocessor air mass flow controllers 216, 218, 220 and stack air massflow controller 64 are closed to seal the inlets, anode bypass valve 26and cathode bypass valve 44 remain in the bypass position, and backpressure regulator 214 remains closed to seal the exit. Recirculationcompressor 32, fuel processor air compressor 202, and stack aircompressor 42 are stopped. The resident gases within fuel processor 12are dry and at an elevated pressure, which is desired for shut downcondition, particularly with a CuZn water gas shift catalyst.

Yet another alternative system is illustrated in FIG. 4 wherein acompressor may be eliminated from the fuel processor system, generallyindicated at 10′″. Fuel processor system 10′″ is operated atsub-atmospheric pressures such that potential for air ingestion exists.Otherwise, the startup, shut down, turn down and transient operation aresimilar to fuel processor system 10″ illustrated in FIG. 3. Anadditional benefit of fuel processor system 10′″ is that a recirculatedexhaust 302 can be made inert by providing just enough cathode exhaust58 to catalytic combustor reactor 16 using a combustor air mass flowcontroller 304 for stoichiometric operation in catalytic combustorreactor 16.

A cathode back pressure regulator 306 is needed to match the pressureset by a back pressure regulator 308 downstream of catalytic combustorreactor 16 to ensure cathode exhaust 58 can be directed to catalyticcombustor reactor 16. An O₂ sensor 310 may be used in exhaust 312 toensure stoichiometric operation.

A unique capability of the aforementioned systems is the potential tooperate without water addition. This is an advantage for a system thatis to be started in ambient temperatures below O° C., where water is notavailable. Because the system 10′″ operates at a high recirculation,this mode of operation is relatively inefficient at about 62%, howeverit may be used for short duration.

It should be understood that features of the fuel processor systemsillustrated in FIGS. 1-4 can be combined as needed for systemrequirements. For example, PrOx air 208 may preferably be delivered fromstack air compressor 42. That is, various combinations of the varioussystems described herein might be made depending upon the specificapplication.

As should be appreciated from the foregoing discussion, the fuelprocessor systems of the present invention all include recirculation offuel processor gases, such as reformate, anode exhaust, or combustorexhaust. This feature provides numerous advantages that are not presentin conventional fuel processor systems. For example, the fuel processorsystems of the present invention are capable of providing a large massflow rate through the fuel processor to aid in heating the fuelprocessor components to the proper operating temperatures duringstartup. Moreover, during shut down, the fuel processor systems of thepresent invention enable the fuel processor to run dry and condensewater from the reformate to avoid condensation on the catalysts andsubsequently be shut down at an elevated pressure to prevent airingestion upon cooling of the fuel processor. Still further, during turndown, the fuel processor systems of the present invention enable higherflow rates through the fuel processor and fuel cell stack to avoidauto-ignition in the inlet, reverse water gas shift in the PrOx, cellreversal in the fuel cell stack, and water collection, in the fuel cellstack, all of which occur at reduced flow rates. During transientresponse, the fuel processor systems of the present invention, bycirculating gases, enables the flow rate and pressure in the fuelprocessor to remain nearly constant, thereby minimizing the lag intransient response associated with filling or venting volumes in thefuel processor system. The ability to use recirculated gases, whichcontain water vapor as a product of reaction, enables the fuel processorto run without water injection. The fuel processor systems of thepresent invention enable rapid thermal start of the fuel processorwithout the complexity of multiple stages or risk of oxygen exposure.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A fuel processing system comprising: a fuel processor having a fuelprocessor inlet and a fuel processor outlet; a combustor having acombustor inlet in fluid communication with said fuel processor outletand a combustor outlet; a recirculation loop including a first valve forselectively providing fluid communication between said fuel processoroutlet and said fuel processor inlet, wherein said recirculation loopprovides fluid communication from said fuel processor outlet throughsaid combustor to said fuel processor inlet, and a fuel cell having ananode inlet in fluid communication with the fuel processor outlet and ananode outlet; wherein the recirculation loop includes an anode bypassvalve operably disposed directly between the fuel processor outlet andthe anode inlet for selectively providing fluid communication betweenthe fuel processor outlet and either the anode inlet or the combustorinlet, thereby bypassing the fuel cell; and a pressure regulatoroperably disposed directly between the combustor inlet and one of theanode outlet or the anode bypass valve.
 2. A fuel processing systemaccording to claim 1, wherein the recirculation loop includes a bypassvalve for selectively providing fluid communication between said fuelprocessor inlet and at least one of said fuel processor outlet and saidcombustor outlet; and further comprising a bypass passage providingfluid communication between said recirculation loop and said combustorinlet comprising a check valve to prevent backflow therein.
 3. A fuelprocessing system according to claim 1, wherein the pressure regulatoris operably disposed between said combustor inlet and said anode outlet.4. A fuel processing system according to claim 1, wherein the pressureregulator is operably disposed between said combustor inlet and saidanode bypass valve.
 5. A fuel processing system according to claim 1,further comprising a recirculation pump in fluid communication with saidrecirculation loop.
 6. A fuel processing system according to claim 1,further comprising: a fuel cell between said fuel processor and saidcombustor having an anode inlet in fluid communication with said fuelprocessor outlet and an anode outlet in fluid communication with saidcombustor inlet; and a recirculation valve for controlling fluidcommunication through said recirculation loop.
 7. A fuel processingsystem according to claim 1, further comprising: a fuel cell betweensaid fuel processor and said combustor having an anode inlet in fluidcommunication with said fuel processor outlet and an anode outlet influid communication with said combustor inlet; and a recirculation pumpin fluid communication with said recirculation loop, wherein saidrecirculation pump is operably disposed between said recirculation loopand said fuel processor inlet, and a pressure regulator operablydisposed between said anode outlet and said combustor inlet.
 8. A fuelprocessing system according to claim 1, further comprising: a fuel cellbetween said fuel processor and said combustor having an anode inlet influid communication with said fuel processor outlet and an anode outletin fluid communication with said combustor inlet; and a cathode exhaustin fluid communication with said combustor inlet and having a checkvalve operably disposed between said cathode exhaust and said combustorinlet to prevent backflow through said fuel cell.
 9. A fuel processingsystem according to claim 1, further comprising: a fuel cell betweensaid fuel processor and said combustor having an anode inlet in fluidcommunication with said fuel processor outlet and an anode outlet influid communication with said combustor inlet; and a recirculation airsupply in fluid communication with said recirculation loop.
 10. A fuelprocessing system according to claim 1, further comprising: a fuel cellhaving an anode inlet in fluid communication with said fuel processoroutlet and an anode outlet in fluid communication with said combustorinlet.